I use small subs for this:

sub undef2blank
{
  return defined($_[0]) ? $_[0] : "";
}

my $outPath = $obj->{outpath}{absolut} . "-" . undef2blank($obj->{sap}
+{jfExtNum});
[download]

It's probaby slower than the other ideas suggested here, but it's obvious what it does and keeps the general style of the original code.